Many applications for lasers, including spectroscopic studies, materials processing, nonlinear optical interactions and information transmission, require laser radiation which is characterized by: a) high power; b) diffraction-limited divergence; and c) narrow bandwidth. Reduction of the divergence and bandwidth of conventional lasers is accomplished by restricting the spatial and spectral modes of the laser radiation, which usually results in decreasing the output power. Methods and apparatus which provide all three characteristics simultaneously are usually costly and complicated, and often require multiple laser sources.
Stable laser cavities, for example, are used to provide radiation having low spatial divergence and narrow bandwidth. Low spatial divergence is obtained by restricting the laser aperture to operate in the lowest transverse mode of the stable cavity. For near infrared or shorter wavelengths, aperture diameters on the order of 1 millimeter (mm) or less are required, while gain medium diameters of several centimeters are common. As a result, the power available from a stable laser cavity is limited, often to values that are orders of magnitudes less than those obtainable if the full aperture of the gain medium could be used. The bandwidth of stable cavities is usually restricted by using a conventional dispersing element, i.e., a grating, a prism or a Fabry-Perot etalon, in the cavity to spread the radiation angularly according to wavelength. Narrow-bandwidth operation is then obtained by restricting the angular acceptance of the cavity. This operation is compatible with a mode-restricting aperture used for limiting the spatial divergence, as described above, but is incompatible with high power operation.
High power laser sources are obtainable with narrow bandwidths and low divergence radiation using laser amplifiers downstream of the resonator. The output power which can be extracted from the stable resonator is severely restricted by the aperture diameter. As a result, one or more stages of amplification are required to increase the output power to a level equal to the gain available from the gain medium. Such systems are the primary source for laser radiation with all three desired characteristics, but the multiple lasers required by these systems increase both the size and cost of the system. In addition, the complexity of the optical train required to match the output of one stage into the input of the next stage increases. For pulsed laser systems, jitter in timing between the various stages can reduce system reliability.
Unstable resonators provide an alternative approach to obtaining high power, low divergence laser radiation. In unstable resonators, the laser radiation fills a relatively large diameter cavity, allowing operation at high power levels while restricting the divergence of the generated laser radiation to a low value, usually near the diffraction limit for a suitably designed system. Some success has been achieved in frequency narrowing the laser radiation from unstable resonator cavities using gratings, which work best with lasers having sharp line structure, e.g., molecular lasers. For example, selection of a single line in HF and CO.sub.2 lasers by insertion of a grating into a conventional unstable resonator cavity is known.
In lasers having broad-band continuous gain distributions, i.e., excimer and dye lasers, insertion of a grating in an unstable resonator cavity does not provide sufficient spectral discrimination for narrow-band operation. The unstable resonator cavity is fundamentally incompatible with the frequency narrowing elements used in restricting the bandwidth, especially when extremely narrow linewidths are desired. The modes of an unstable resonator cavity require that divergence of the laser radiation inside the cavity alternate between high and low values on alternate passes through the cavity. As a result, it is not possible to use angular discrimination to restrict the bandwidth of the laser radiation as is done in stable cavities. Thus, although unstable resonator cavities are the configuration of choice for providing high power, low divergence laser radiation, they are usually not compatible with a simultaneous requirement for narrow bandwidth.
An unstable resonator cavity employing a telescopic full cavity ring, in which the gain medium and the ring form a continuous loop and the magnification is achieved by a telescope within the ring, is also known. This cavity has extensive collimated regions that offer the potential for frequency narrowing. However, the beam passes through the gain medium only once on each cavity round trip. As a result, this type of cavity generally requires a large number of cavity transits to reach threshold and can work only with lasers that have gain media with a combination of high gain and long lifetime.
An unstable resonator cavity laser developed by some of the inventors of the present invention is disclosed in U.S. Pat. No. 4,868,515, which achieves all three desirable characteristics of high power, low divergence and narrow bandwidth by employing an asymmetric feedback ring. This invention represents a substantial improvement in the state of the art because the feedback ring provides a section of the cavity that contains only collimated laser radiation, thus allowing optimal use of frequency narrowing devices, e.g., Fabry-Perot etalons. At the same time, the length of the feedback ring can be kept to a minimal value, and the laser radiation makes two passes through the gain medium on each cavity transit, thus overcoming the limitations of the cavity with the full telescopic ring. As a result, this cavity works with all types of lasers, including the class of lasers in which the gain medium has limited gain or lifetime, such as electric discharge rare gas halide excimer lasers, with which the resonator with the full cavity ring would not work.
An alternative approach for producing high power laser radiation with low divergence and narrow bandwidth is to couple together a stable and an unstable resonator. In one existing system, laser radiation from the stable resonator passes through a hole in a mirror of the unstable resonator, which is located within the stable resonator. Laser radiation coupled into the unstable resonator is reflected from the mirror surrounding the hole. Thus, only the outer edge of the laser radiation within the stable resonator is coupled into the unstable resonator. The outer edge is the part of the laser radiation most sensitive to imperfections in the diffraction structure. In addition, both the fraction of laser radiation coupled out of the stable resonator and the purity of the mode within the stable resonator are determined by the size of the hole. As a result, it is not possible to simultaneously optimize the operation of both the stable and unstable resonators for mode structure and power.
Other techniques for coupling stable and unstable resonators are known. One approach, for example, uses two mirrors with different curvatures so that the resonator is stable in one direction but unstable in the opposite direction. Such techniques do not produce laser radiation with the low divergence required for efficient frequency narrowing. Another approach uses two separate cavities operating with a common mirror, producing all the disadvantages of multiple laser stages discussed above.